1. Field of the Invention
The present invention generally relates to an optical measuring apparatus and, more specifically, to an optical measuring apparatus provided with a lighting unit having a light-emitting diode as a light source.
2. Description of the Related Art
An optical measuring apparatus provided with various lighting units (for example, an incident-light lighting unit or a transmission lighting unit) mounted on a measuring apparatus body and configured to measure the shape or the like of a measured object (work) while irradiating the measured object with light from any one of the lighting units is known.
For example, an image processing measuring apparatus described in Japanese Unexamined Patent Application Publication No. 2004-220834 includes an LED (Light Emitting Diode), a CCD (Charge Coupled Device) camera (image pickup device), and a control device configured to control the LED and the CCD camera. The control device illuminates the measured object while controlling an applied current to the LED according to an inputted light intensity command value, and acquires image information by controlling the CCD camera and receiving reflected light from the measured object. Then, the shape of the measured object is measured by processing the acquired image information.
In the optical measuring apparatus, differences in brightness affects measured results, and hence it is necessary to cause the lighting unit to generate an accurate light intensity corresponding to the given light intensity command (target) value. However, in an optical measuring apparatus having a lighting unit with an LED as a light source, the light intensity varies from LED to LED, even when the LEDs have the same model number. Therefore, a countermeasure for generating predetermined light intensities corresponding to the light intensity command values must be employed.
For example, as shown in FIG. 7, a conversion table (calibration value) including current command values and PWM (pulse width modulation) duty ratios required for causing an LED 44 of a lighting unit 4 to generate a light intensity corresponding to a light intensity command value (0 to 100%) given from the measuring apparatus body or a PC (Personal Computer) 3 is stored in a storage unit 48 of a lighting controller 6.
In addition to the storage unit 48, the lighting controller 6 is provided with a Digital to Analog (DA) converter 61, a Pulse Width Modulation (PWM) generator 62, a control unit 63 configured to read the current command value and the PWM duty ratio corresponding to the light intensity command value out from the storage unit 48 upon receipt of the light intensity command value and set the current command value and the PWM duty ratio to the DA converter 61 and the PWM generator 62, and a constant current generator 64.
Upon receipt of the light intensity command value, the control unit 63 reads calibration values (current command value and PWM duty ratio) corresponding to the light intensity command value out from the storage unit 48, and sets the current command value and the PWM duty ratio to the DA converter 61 and the PWM generator 62. A control pulse of the set PWM duty ratio is generated from the PWM generator 62. Then, when the control pulse generated from the PWM generator 62 is ON, the constant current generator 64 applies a current converted by the DA converter 61 corresponding to the current command value to the LED 44 of the lighting unit 4. Accordingly, the LED 44 emits light at the light intensity corresponding to the light intensity command value.
In the optical measuring apparatus in the related art, since the calibration values are stored in a lighting controller separate from the lighting unit, there are several problems.
Generally, the lifetime of the LED is longer than fluorescent lamps or the like, but the brightness is lower. Therefore, in applications which require accurate brightness, replacement of the LED at regular time intervals is desired. Accordingly, when replacing the lighting unit, it is necessary to write the calibration values to the lighting controller.
Writing the calibration values to the lighting controller is an important setting that affects the measurement accuracy, and must be performed with a high degree of reliability. Normally, a serviceman of the manufacturer needs to go to a user's site to perform the setting operation.
Further, registration of the calibration value at the time of assembly can be performed only after a combination of the lighting controller and the lighting unit is determined.
Therefore, the measuring apparatus in a latter half of assembly, and being almost completed, must wait for a time-consuming light calibration operation, resulting in disadvantages including an increase in the number of components in the midcourse of assembly and lowering of the manufacturing space efficiency.
A counter measure such that the time-consuming light calibration is performed in advance and the calibration data is stored in an FD (flexible disk) is also conceivable, but managing the FDs and the lighting units in pairs is troublesome.
When a wrong measuring unit is connected, that is, when a lighting unit other than the predetermined lighting unit is connected to the lighting controller, the apparatus may be used with an incorrect brightness.